Antiskid brake controllers have been in widespread use for many years. In the simplest sense, an antiskid brake controller compares the speed of a vehicle (e.g., automobile, aircraft, etc.) derived from a wheel speed sensor to the vehicle speed derived from a secondary or reference source. If the wheel is determined to be slipping an excessive amount (i.e., skidding), then brake pressure applied to the wheel is released and the wheel is allowed to spin back up to the appropriate speed.
A fundamental problem associated with virtually all antiskid brake controllers relates to determining an appropriate amount of slipping. Two types of controllers which are generally known utilize different techniques. The first type of antiskid controller is deceleration based. In short, the deceleration based controller differentiates the wheel speed to determine how fast the wheel speed is changing. If the wheel decelerates too quickly, there is said to be excessive skidding and the controller reduces the amount of pressure transmitted to the brakes.
A second type of antiskid controller relies on a model of the mu-slip curve which describes the tire-to-road surface friction characteristics. The difference between the wheel velocity and the vehicle velocity is referred to as the slip velocity. The slip velocity is compared with a predefined set point on the mu-slip curve in order to achieve a desired amount of slip.
In the case of an aircraft, the antiskid controller is intended to prevent tire skidding during a landing event or during taxi operations. A skidding tire has at least three detrimental effects which are commonly known. For example, drag at the wheel/runway surface interface is lost and the length of the runway needed for the aircraft to stop is increased. Moreover, lateral (or side) wheel friction is reduced during a deep skid, so the ability of the pilot to steer the aircraft is diminished. Finally, tire life is reduced when the wheel skids because patches of rubber are torn from the tire.
A wheel slips (i.e., the rotational speed times the wheel radius is less than the translational speed of the wheel) whenever brake torque is applied to a rotating wheel. The amount of slip determines the drag that is produced at the tire/runway surface interface. There is an optimum amount of slip that generates a maximum tire drag as defined by the well known mu-slip curve. A wheel experiencing slip beyond the optimum amount is considered to be excessively skidding.
Antiskid controllers which are based on the mu-slip curve are oftentimes computationally complex and require multiple sensors for measuring wheel speed, vehicle speed, etc. Consequently, there is a need for an antiskid controller which is not computationally complex and which preferably can provide control with as little as a single measurement sensor. For example, there is a strong need in the art for a mu-slip based antiskid controller capable of providing antiskid control using wheel speed as its only measured parameter.